1. Field of the Invention
This invention relates to a method of efficiently recovering gold and silver from a silicate ore.
2. Description of the Prior Art
There are two main methods known for smelting a silicate ore containing at least 0.1 g of each of gold and silver per ton. One of them is a hydrometallurgical method which is intended only for the recovery of gold and silver. The other is a pyrometallurgical method in which the ore per se is used as a flux for the pyrometallurgical smelting of copper or lead, while gold and silver are recovered as by-products.
Slurry cyaniding is a widely used hydrometallurgical method, but the disposal of the waste solution and tailings which contain cyanogen is a big problem. The thiourea method and flotation are hydrometallurgical methods which do not use any cyanogen solution. The former is, however, still under study and not yet employed in practice (Studies and Prospects of Gold Extraction from Carbon Bearing Clayey Gold Ore by the Thiourea Process, Chen Deng Wen: Preprints--The Canadian Institute of Mining and Metallurgy, XIV International Mineral Processing Congress, Toronto, Canada, Oct. 17-23, 1982, II--8.1 to II--8.11). The latter can only recover gold and silver at a low yield (about 93% in the case of gold--The Japanese Association of the Mining Industry: Report on the Principal Costs of Flotation and Cyaniding, 1981).
The method in which the ore is used as a flux in, for example, a flash smelting furnace or converter for copper has the disadvantage that the resulting slag has an Fe (%)/SiO.sub.2 (%) ratio which is limited by its physical properties. The ratio is about 1.0 in the case of a slag from a flash smelting furnace and about 1.8 to 2.0 in the case of a converter slag. Any attempt to melt the silicate ore until a lower FeO/SiO.sub.2 ratio is reached gives rise to a great increase in the viscosity and melting point of the slag (The Japan Institute of Metals: Nonferrous Smelting, p. 63), resulting in an increased loss of copper to the slag. It is necessary to raise the molten bath temperature in order to cope with any such increase in the viscosity and melting point of the slag. It is, however, practically difficult to employ a higher molten bath temperature, since it promotes the erosion of the refractories. The converter slag usually contains 25 to 35% of Fe.sub.3 O.sub.4. Even a small increase of SiO.sub.2 greatly raises the melting point of the slag. A part of the silicate ore remains undissolved with a resultant reduction in the primary yield of recovered gold and silver.